The invention relates generally to an apparatus and method for converting AC voltage to a variable DC voltage and more specifically to an apparatus for converting AC voltage to a variable DC voltage for exciting the field windings of electrical generating equipment.
It has been common practice to use a full thyristor bridge to convert AC voltage to a variable DC voltage for exciting the rotating field of a synchronous generator. A typical example of an exciter 10, employing such a full thyristor bridge, is shown in FIG. 1. The full thyristor bridge 100 includes six thyristors 101-106, one connected in each of six legs of the bridge. A full thyristor bridge circuit includes many advantages such as the ability to transiently invert field voltage to rapidly decrease generator field flux linkages.
Conventional phase-controlled thyristor power converters (“bridges”) have been used in DC motor drive and generator excitation supplies for several decades. The thyristors (“SCRs”) used in these bridges have electrical specifications such as peak voltage and maximum rate of change of voltage (dv/dt) and maximum rate of change of current (di/dt), which must not be exceeded in order to ensure reliable operation. With respect to maximum di/dt, a thyristor requires time to distribute current conduction uniformly throughout the junctions. If the rate of rise of anode current is very fast, localized heating will occur due to high current density, resulting in excessive temperature and possible failure of the device. Exceeding maximum dv/dt across a thyristor may cause the device to turn on without a gating signal at an unintended time.
Various components are added to the bridges to meet these specifications. An RC snubber is often placed in parallel with each thyristor to control the dv/dt and voltage overshoot. An inductor (“ferrite”) may be placed in series with each thyristor to control the di/dt. In addition to the RC filter across the SCR, sometimes a three-phase delta RC filter (“AC line filter”) is connected across the incoming power source. This filter has the benefit of reducing the size of the RC snubber across the SCR. Further, balancing or peak repetitive voltage (PRV) resistors may be placed from each AC input line to each of the positive and negative DC output lines. The PRV resistors balance voltage across thyristor cells from the positive DC bus to the negative DC bus during the off state. The PRV resistors also permit a higher pole slip voltage without exceeding peak cell voltage. An RC filter may also be placed across the thyristor bridge DC output to limit peak voltage and dv/dt to the load. In excitation systems this is often referred to as a shaft voltage suppressor (SVS).
Referring again to FIG. 1, each of the six thyristors incorporates a snubber circuit 110, including snubber capacitor 115 and snubber resistor 120 in parallel with each thyristor 101-106. Leg ferrites 125 may be provided in each leg of the three-phase bridge 100. Line ferrites 130 may be provided on the input line to each phase of the three-phase bridge 100.
FIG. 1 also illustrates a three-phase AC line filter 170. Each leg of the three-phase AC line filter 170 includes a parallel combination of a series filter resistor 180 and series filter capacitor 185 with a discharge resistor 190. The three-phase AC line filter 170 is typically designed to absorb transients from the AC line power source 140 and from the bridge 100.
An output shaft voltage suppressor 195 providing voltage transient absorbing circuits 197 with RC elements 199 is further shown in FIG. 1. The DC output of the three-phase thyristor bridge in input is input to field windings 193 of the electrical generating equipment (not shown) being supplied.
All of these filtering components together typically exceed the cost of the thyristors themselves. Excitation bridges may be supplied with all of these components, for a number of reasons. Early thyristors were fragile and needed good protection, so significant conservatism was incorporated in the design. Bridges designed for more stringent applications such as common transformer motor drives have been directly used for exciters. Further designs may be set conservatively due to the high reliability and availability requirements of the electric power generation industry. Excitation bridges by GE® have typically incorporated all the above described protective features. Incorporation of all the above protective features provides reliable operation and highly protected components, but results in increased material and labor costs.
During the transition of an SCR 101-106 from conducting to blocking, there is a charge in the SCR 101-106 which much be removed before the SCR 101-106 can block reverse current. This energy typically goes into the snubber capacitors 120, the AC line filter capacitors 185, or a bucket suppressor capacitor 280 (FIG. 2). If this charge is not absorbed by a component such as these, high voltage and dv/dt transients occur in the exciter or associated external equipment such as the generator field or the source transformer.
Bucket filters have been used on certain exciters in place of snubbers, ferrites and the three-phase AC line filter. In these applications, the bucket filter is employed in an attempt to absorb the energy that otherwise would have been absorbed by the snubbers across each thyristor.
A typical bucket filter is illustrated in FIG. 2. The bucket filter 200 includes a three-phase diode bridge 205 having six legs across an AC source 140. Each bridge leg 215 includes two series diodes 220 (or may be a single diode) in parallel with two series resistors 230, the mid-point of the series diodes 220 and series resistors 230 being tied together. The three-phase diode bridge 205 outputs to a single RC output filter 270, including filter resistor 275 and filter capacitor 280. Bleed resistor 290 is connected in parallel to filter capacitor 280.
The three-phase diode bridge 205 automatically switches the single RC filter 270 to the AC line being commutated in the thyristor bridge (not shown). Single RC filter 270 protects the thyristor being commutated by absorbing the overvoltage peaks and limiting dv/dt on the thyristor. Utilization of series diodes 220 provides some voltage margin and some degree of redundancy in providing a path to the overvoltage protection afforded by single RC filter 270. Series resistors 230 force voltage sharing across series diodes 220. Bleed resistor 290 dissipates thyristor recovery energy absorbed by filter capacitor 280 of single RC output filter 270.
Evaluation of the bucket suppressor approach, used in some designs especially in Europe, revealed several disadvantages. This approach removes all snubbers and relies on a single capacitor to absorb recovery energy from all thyristors. This results in high stress in the capacitor. The diode bridge also sees high peak currents. Sizing the diode bridge to handle the high voltages and current peaks can make this an expensive approach. In addition, the diodes also have recovery charges, which result in additional transient disturbances to the system. Reliability is lower due to the higher failure rates of active diodes compared to passive designs.
Accordingly, there is a need to provide an exciter circuit that delivers reliability and adequate margin to limits for components, but which also reduces complexity, components and cost.